Concrete block calculations

1) Multiply the length of the footing in feet by 3/4 to get the number of blocks per row.

2) Divide the height of the wall in inches by 8 to get the number of courses.

3) Multiply the two answers to get the number of blocks.

4) If the wall is 10" or 12" block, multiply the number of corners X the number of courses to get the number of corner blocks.

5) Subtract the corner blocks from the plain blocks to get the correct number of plain (stretcher) blocks.

6) Where the height of the wall doesn't work out to an even number of courses, you can fiddle with the thickness of the mortar joint or use a course of ashlars -- half-height blocks.

Example

Let's take a 20' X 16' addition and assume a 6'-high wall of 10" block.

1) 20 + 20 + 16 = 56 X .75 = 42 blocks/course.

2) 6' X 12" = 72"/8 = 9 courses.

3) 42 x 9 = 378.

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4) Two outside corners X 9 courses = 18 corner blocks.

5) 378 - 18 = 360 blocks.

Calculating mortar

1. The amount of sand and cement required to lay any size of block is pretty well the same.

Each block requires .003 cu. yd. of sand. Each cu. yd. of sand requires 12 bags of masonry cement.

Or, each block requires .162 bag of mortar mix.

2. Mix one part cement, 3 parts sand. Use as much water as you can and still get a workable mix because the blocks absorb the excess water.

3. The amount required to parge is about equal to the amount required to lay, so if you are laying and parging, double the amount calculated to lay only.)

Example

For our wall, we need .003 X 378 = 1.1 cu. yd sand. We also need 12 X 1.1 = 13 bags of masonry.

Assume we are parging most of the way up and at least half the wall will be below grade. Order 2 cu. yds. sand, 10 bags type S masonry, and 10 bags type N masonry.

Or we could use .162 X 378 X 2 = 122 bags mortar mix.

(Some old pros like to use portland and sealbond to make a custom mix which works particularly well for parging or stucco work because you can control the plasticity of the mix better. The more sealbond the more workable, the more portland the stronger.)

Tips

1) If hand-mixing, save some work. Make a really sloppy mix with all the cement but short on sand. Only mix it a little bit. Leave it for 10 minutes, the water will soak into the mix and when you come back to it you can finish mixing it, and add sand to bring it to the right consistency.

2) If using the mortar for tuck pointing, temper it by thoroughly mixing it, then letting it sit for half an hour, then re-mix it. This reduces the chance of it shrinking in the joint.

3) Contrary to all recommendations, I have found that you can use mortar in freezing weather and still get a very strong wall. I think the heat of hydration is enough to make it cure, but the cold slows it down so it cures rather than drying out as it does in the summer. The strongest wall I ever built was to be temporary so I wasn't concerned that the mortar had ice crystals in it as I laid the blocks. The next Spring, I had to smash the blocks themselves, and there was no hope of salvaging them for re-use.